UNIVERSITY OF NAIROBI

SCHOOL OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING

DESIGN A THREE PHASE CONTROLLED RECTIFIER FOR USE IN THE

LABORATORY

PROJECT INDEX: 116

SUBMITTED BY

MWONGA BRIAN MATOLO

F17/39946/2011

SUPERVISOR: MR. S. L. OGABA

EXAMINER: MR OMBURA

PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE

REQUIREMENT FOR THE AWARD OF THE DEGREE

OF

BACHELOR OF SCIENCE IN ELECTRICAL AND ELECTRONIC ENGINEERING OF

THE

UNIVERSITY OF NAIROBI

DATE SUBMITTED: 17TH MAY 2016

ii

DECLARATION OF ORIGINALITY

NAME OF STUDENT: MWONGA BRIAN MATOLO

REGISTRATION NUMBER: F17/39946/2011

COLLEGE: Architecture and Engineering

FACULTY/SCHOOL/INSTITUTE: Engineering

DEPARTMENT: Electrical and Information Engineering

COURSE NAME: Bachelor of Science in Electrical and Electronic Engineering

TITLE OF WORK: Three Phase Controlled Rectifier For Use In The Laboratory

1) I understand what plagiarism is and I am aware of the university policy in this regard

2) I declare that this final year project report is my original work and has not been submitted

elsewhere for examination, award of a degree or publication. Where other people’s work or my

own work has been used, this has properly been acknowledged and referenced in accordance

with the University of Nairobi’s requirements.

3) I have not sought or used the services of any professional agencies to produce this work.

4) I have not allowed, and shall not allow anyone to copy my work with the intention of passing

it off as his/her own work.

5) I understand that any false claim in respect of this work shall result in disciplinary action,

in accordance with University anti-plagiarism policy.

Signature…………………………………………………………………..

Date………………………………………………………………………..

iii

CERTIFICATION

The report has been submitted to the Department of Electrical and Information Engineering,

University of Nairobi with my approval as supervisor:

Mr. S. L. Ogaba.

Date: ……………………………..

i

DEDICATION

I dedicate this project to my parents and family for always believing in me and to my friends

for their encouragement and support.

ii

ACKNOWLEDGEMENT

I wish to appreciate the Almighty God for His amazing grace throughout my life. His love and

guidance has brought me this far to complete my final year project entitled “Three phase

controlled rectifier for use in the lab”.

I would also like to take this opportunity to thank all the people who had assisted me directly

and indirectly in completing the project. My first gratitude goes to Mr. S.L Ogaba my

supervisor for the project whom had given support, advice and Guidance. I am indebted to him

for always being there to help me shape the problem and provide insights towards the solution.

The progress I have made now would have been impossible without his guidance. I also

appreciate the teaching staff and non-teaching staff at the University of Nairobi, Department

of Electrical and Information Engineering for their selfless effort that enabled me to achieve

my academic goals during the entire course of my studies.

I would also like to thank my friends for their constant encouragement and helpful discussions

throughout the course of this work.

Last but not least, I would like to thank my beloved parents who had given me a lot of moral

support while I was doing this project.

Once again to all, thank you very much.

iii

ABSTRACT

Three phase AC to DC conversion of electric power is widely employed in adjustable speed

drives (ASDs), uninterruptible power supply (UPSs), HVDC systems and utility interfaces with

non-conventional energy sources such as electroplating welding units etc, battery charging for

electric vehicles and power supplies for telecommunication systems.

The purpose of this project is to successfully convert three phase ac to dc conversion of electric

power using SCRs. AC to DC line commutated converters are used as the most usual choice

for application. This is due to simplicity of the circuits requiring a minimum number of active

and passive components. Thyristors or SCRs are the main line commutated power switches. In

this project is built on thyristors and a control or firing system using ATmega328p

microcontroller which is used to trigger the three phase full-controlled bridge rectifier.it adopts

three synchronization signals; 6-channels trigger pulses from it are very

symmetrical and the pulses can be shifted within a cycle of a sine wave from 0 degree to 162

degrees

Its structure of this circuit is simple and consists of the rectifier circuit consisting of the 6-SCRs

and the isolation circuit with the protection system, a microcontroller a LCD screen

,synchronization circuits and the control voltage circuit.; its performance is high stable and

reliable; Its anti-interference ability is strong and its operation is easy.

A zero crossing circuit attached to the AC lines produces a digital signal to the processor (based on

signal zero crossings) that is captured and used to predict the next zero crossing capture time

and fire the SCR’s based on this prediction. For accuracy and precision, a phase locked loop

algorithm is implemented in software that works to improve the accuracy of the next predicted

zero crossing and locks to produce the desired output voltage.

iv

Table of Contents

DECLARATION OF ORIGINALITY ............................................................................................... ii

CERTIFICATION ............................................................................................................................... iii

DEDICATION........................................................................................................................................ i

ABSTRACT .......................................................................................................................................... iii

LIST OF ABBREVIATIONS ............................................................................................................... vii

CHAPTER 1 .......................................................................................................................................... 1

1.1 Overview .......................................................................................................................................... 1

1.2 background .................................................................................................................................. 3

1.3 Objectives..................................................................................................................................... 4

1.4 Scope ............................................................................................................................................ 4

1.5 problem statement ...................................................................................................................... 4

1.6 Project Report outline ................................................................................................................ 5

CHAPTER 2 .......................................................................................................................................... 6

LITERATURE REVIEW .................................................................................................................... 6

2.1 Introduction ................................................................................................................................. 6

2.2 Theory of rectifiers ..................................................................................................................... 7

2.2.1 Single-Phase Rectifiers ...................................................................................................... 10

2.2.2 Three Phase Rectifiers ....................................................................................................... 13

2.3 Thyristor commutation — effect of the AC input reactance ................................................ 18

2.4 Power displacement factor and Power factor ................................................................... 21

2.5 Effects on the AC mains voltage .............................................................................................. 22

2.6 How to reduce the harmonics on the AC mains ................................................................... 23

2.6.1 Three-Phase Line Reactors ............................................................................................. 23

2.6.2 DC-Link Choke .................................................................................................................... 24

2.6.3 Passive Filters ..................................................................................................................... 24

2.7 Silicon Controlled Rectifier ...................................................................................................... 26

2.7.1 Working of SCR ................................................................................................................. 27

2.7.2 Switching Characteristics of SCR..................................................................................... 28

2.7.3 SCR Ratings ....................................................................................................................... 29

2.7.4 Gate Circuit Requirements. .................................................................................................. 29

2.7.5 SCR protection ................................................................................................................... 31

2.8 Previous researches ...................................................................... Error! Bookmark not defined.

CHAPTER 3 ........................................................................................................................................ 32

METHODOLOGY ............................................................................................................................. 33

3.1 introduction ............................................................................................................................... 33

v

3.2 Design ......................................................................................................................................... 34

3.2.1 Input .................................................................................................................................... 34

3.2.2 The rectifier circuit ............................................................................................................ 35

3.2.3 Synchronization circuit/ Zero crossing detector circuits. ............................................... 38

3.3.3 Controlling voltage ............................................................................................................. 39

3.3.4 Generating of the firing pulses .......................................................................................... 40

3.4 Software Design ......................................................................................................................... 43

3.4.1 Main Program .................................................................................................................... 43

3.4.2 ADC program for delay ..................................................................................................... 45

CHAPTER 4: ....................................................................................................................................... 46

RESULTS AND ANALYSIS ............................................................................................................. 46

4.1 Theoretical Results .................................................................................................................... 46

CHAPTER 5 ........................................................................................................................................ 47

CONCLUSION AND RECOMMENDATION ............................................................................ 47

5.1 Conclusion .................................................................................................................................. 47

5.2 Recommendations for Further Work ..................................................................................... 47

REFFERENCES ................................................................................................................................... 48

vi

TABLE OF FIGURES

Figure 1: Rectifier ................................................................................................................................... 2

Figure 2: inverter..................................................................................................................................... 2

Figure 3: Chopper ................................................................................................................................... 2

Figure 4 cycloconverter ........................................................................................................................... 3

Figure 5 :power electronics interface between the power processor and controller ............................ 4

Figure 6:uncontrolled rectifier and sub divisions ................................................................................... 8

Figure 7 : controlled rectifier and sub divisions ...................................................................................... 9

Figure 8: Single-phase half wave diode rectifier-R/L Load ................................................................... 10

Figure 9: A basic half wave controlled rectifier .................................................................................... 11

Figure10 a: The Bridge Rectifier Figure 10 b: single diode rectifier waveforms ............................. 11

Figure 11: a full wave controlled bridge rectifier .................................................................................. 12

Figure 12: single way rectifier ............................................................................................................... 13

Figure 13: waveforms for the number of pulses of single way rectifier ............................................... 14

Figure 14: Three - phase bridge rectifier (a) and (b) it’s ideal equivalent circuit ................................. 14

Figure 15(a): source and output voltage............................................................................................... 15

Figure 16 b: waveforms of voltages and current of a three phase rectifier operating in the constant

load current mode ................................................................................................................................ 15

Figure 17: (a) fully-controlled three phase bridge ciruit, (b) the output voltage waveforms with small

fring angle and (c) with large firing angle ............................................................................................. 18

Figure 18: Commutation of thyristor from one phase to another ....................................................... 19

Figure 19: Effect of the overlap angle on the voltage and currents ..................................................... 20

Figure 20: two- transistor model of the thyristor (SCR) (a and b) and its symbol ................................ 27

Figure 21: layer diagram of thyristor .................................................................................................... 27

Figure 22: The static V-I characteristic of an SCR.................................................................................. 28

Figure 23: (a) Firing network desirable network connection,(b) firing network load line ................... 30

Figure 24: Three phase controlled rectifier circuits .............................................................................. 33

Figure 25: Tri-phase control rectifier block diagram ............................................................................ 34

Figure 26: The input circuit to rectifier ................................................................................................. 35

Figure 27: three phase connected to the supply with the snubber circuits and isolation circuit ........ 36

Figure 28: MOC3021 isolator connection to SCR and interface with ATmega328p microcontroller ... 37

Figure 29: zero crossing circuit with input transformer ....................................................................... 38

Figure 30: square wave zero crossing signals of the two phases ......................................................... 39

Figure 31: Controlling Voltage circuit ................................................................................................... 40

Figure 32: MCU and LCD display ........................................................................................................... 43

Figure 33: waveform of the output pulse ............................................................................................. 43

Figure 34: flow chart ADC subroutine and delay control ..................................................................... 45

vii

LIST OF ABBREVIATIONS

AC - Alternate Current

DC - Direct Current

SCR - Silicon Control Rectifier

BJT – Bipolar junction transistors

GTO - Gate Turn-Off Thyristors

VCD - Variable speed drive

THDI - Total Harmonic Distortion Current

MOSFET - Metal–Oxide–Semiconductor Field-Effect Transistor

HPF - High Power Factor

ADC – Analog Digital Converter

IC - Integrated Circuit

RF - Ripple Factor

TUF- Transformer Utilization Factor

ISR -interrupt service routine

PCINT- Pin change interrupt

MCU-Microcontroller

LCD-Liquid Crystal Display

1

CHAPTER 1

INTRODUCTION

1.1 Overview

Power electronics refers to control and conversion of electrical power by power

semiconductor devices wherein these devices operate as switches Power electronics

circuits convert electric power from one form to another using electronic devices.

Power electronics circuits function by using semiconductor devices as switches, thereby

controlling or modifying a voltage or current. [3]

Power electronics started with the development of the mercury arc rectifier. Invented

by peter cooper 1902, it was used to convert alternating current (AC) into direct current

(DC).From the 1920’s on Uno Lamm developed a mercury valve with grading

electrodes making them suitable for high voltage direct current power transmission. .

By late 1940 Shockley's invention of the bipolar junction transistor (BJT) improved the

stability and performance of transistors, and reduced costs and higher power

semiconductor diodes became available and started replacing vacuum tubes.

The invention of the integrated circuit(IC) by J.Kibly from “Texas Instruments” in

1958 followed by the planar process of “Fairchild Semiconductor” in 1959 that became

the key solid state electronics. The period of power semiconductors began in 1956,

when the silicon controlled rectifier (SCR) was invented by General Electric, greatly

increasing the range of power electronics applications.

The introduction of SCRs resulted into significant improvements in

semiconductor fabrication technology and physical operation were made and many

different types of power semiconductor devices appeared. The growth in power

electronics in the last 15 years application has been remarkable because of this

introduction of very fast and high-power switching devices, like the metal-oxide

semiconductor field-effect transistors (MOSFET), bipolar NPN and PNP transistors,

junction transistors and gate turn-off (GTO) thyristors were developed. Later the

microprocessors, specified integral circuits and power integral circuits were produced.

In the last 20 years, the growth in power electronics application has been

remarkable because of this introduction of very fast and high-power switching devices,

coupled with the utilization of state-of –the –art control algorithms. An electric power

can be converted from one form to another form by using power electronics devices

.The function of power electronics circuits by using semiconductor devices as switch is

modifying or controlling a voltage. The goal of power electronics circuits are to convert

electrical energy from one form to another, from source to load with highest efficiency,

high availability and high reliability with the lowest cost, smallest size and weight and

are hence called converters. They serve as interface between source and load.

Converters are classified by the relationship between input and output.

2

Figure 1: Rectifier

AC to DC converters- Rectifiers that transform ac to DC with adjustment of voltage and current

Figure 2: inverter

DC to AC converters- inverters that produce AC of controllable magnitude and frequency,

particularly with galvanic isolation via a transformer

Figure 3: Chopper

DC to DC converters -conversion of an unregulated dc voltage to a regulated dc voltage, linear

regulators and switching choppers.

3

Figure 4 cycloconverter

AC to AC converters-conversion of an AC power source from one amplitude and frequency

to another amplitude and frequency

1.2 background

Power electronic converters can be found wherever there is a need to modify a form of electrical

energy (i.e. change its voltage, current or frequency). The power range of these converters is

from some milliwatts (as in a mobile phone) to hundreds of megawatts (e.g. in a HVDC

transmission system). With classical electronics, electrical currents and voltage are used to

carry information, whereas with power electronics, they carry power. Thus, the main metric of

power electronics becomes the efficiency. The first very high power electronic devices were

mercury arc valves. In modern systems the conversion is performed with semiconductor

switching devices such as diodes, thyristors and transistors. In contrast to electronic systems

concerned with transmission and processing of signals and data, in power electronics

substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the

most typical power electronics device found in many consumer electronic devices, e.g.

television sets, personal computers, battery chargers, etc. The power range is typically from

tens of watts to several hundred watts. In industry the most common application is the variable

speed drive (VSD) that is used to control an induction motor. The power range of VSDs start

from a few hundred watts and end at tens of megawatts. Power electronics refers to control and

conversion of electrical power by power semiconductor devices wherein these devices operate

as switches. Advent of silicon-controlled rectifiers, abbreviated as SCRs, led to the

development of a new area of application called the power electronics. Prior to the introduction

of SCRs, mercury-arc rectifiers were used for controlling electrical power, but such rectifier

circuits were part of industrial electronics and the scope for applications of mercury arc

rectifiers was limited. Once the SCRs were available, the application area spread too many

fields such as drives, power supplies, aviation electronics, high frequency inverters and power

electronics originated.

4

Figure 5 :power electronics interface between the power processor and controller

1.3 Objectives

The first objective of this project is to design a three phase controlled rectifier circuit to

convert three phase 110V AC input voltage to DC output voltage.

The second objective is to develop a hardware of three phase controlled rectifier with line

input voltage of 110v using six SCRs to convert AC input to a DC output depending on

the firing angle of the SCRs and a control circuit for firing angle to range between 0 ̊ to

160 ̊ degrees varying the DC output voltage and can be used by students in the lab to study.

1.4 Scope

This project concentrates on design of circuit and development of hardware to get a DC output

at a three phase AC input. The three phase controlled rectifier uses six SCRs with protective

circuits and filter circuits in the rectifier circuit and AVR Atmega328p microcontroller and a

synchronizing circuit (zero crossing detector circuit) in the control circuit. Besides the scope is

to program a microcontroller to control delay angle alpha 0 ̊ to 160 ̊ to rectifier AC voltage DC

voltage.

To develop the whole project, it consists of three methods which are the concept of switching,

electrical structure, and the software programming.

After designing and building completely the rectifier circuit, the driver circuit should be able

to control the delay angle α. The delay angle will be adjusted by using microcontroller AVR

Atmega328p. It will involve the programming development to control the ON state of the SCRs

and adjust the phase angle. Here, the trigger angle of SCRs will be programmed in certain time

sequence to ensure the input voltage goes from low to full voltage on which the circuit is to be

connected to different loads or supply dc voltage.

1.5 problem statement

A rectifier is a power electronic circuit converter that converts bidirectional AC voltage

to unidirectional DC voltage by using power diodes or by controlling the firing angle of

5

thyristors or controllable switches. Rectification is of two forms uncontrolled rectification

where there is use of power diodes and controlled rectification by use of thyristors or

controllable switches.it can be either single phase or three phase and this depends on the

input. A diode is the simplest electronic switch. It is uncontrollable in that the on and off

conditions are determined by voltages and currents in the circuit. The diode is forward-

biased (on) and reverse-biased (off).

AC to DC converter is mostly used in industries and also in domestic equipment.ac

voltage from generators are variable hence provide variable dc if uncontrolled AC-DC

converter is used, this variable DC is not suitable to be used in the lab for voltage supply

purposes .

To overcome this problem the output of the voltage of the rectifier need to be controlled.

In the design of the three phase rectifier thyristors or SCRs (Silicon Control Rectifier) are

connected in full wave rectifier. Thyristors are electronic switches used in power electronic

circuits where control of switch turn-on is required and controlled by a control system for

which the output voltage can be varied by the user to desired voltage.

Power converter system was designed using modern technology and methods to implement a three

phase fully-controlled rectifier.

1.6 Project Report outline

This project report is structured into five chapters and every chapter elaborates into details

more about this chapter.

Chapter one is the introduction of this project which gives the overview of this project.

Converters of power by providing background information. The three phase controlled rectifier

ac to dc converter is discussed including the objectives, scope and problem statement as a guide

to develop the three phase controlled rectifier.

The second chapter is composed of the literature review of the three phase controlled rectifier.it

discusses the previous research done by others and the comparison with the relevance the

research. Discussion of the books and papers that are used in the reference and the shortcomings

of the previous research.

Chapter three explains and discusses on the methodologies of the three phase controlled

rectifier that has been used and applied in the design and development of the hardware.it also

consists of block diagram, flow charts and circuit diagrams which are explain the process of

implementation and how the three phase AC voltage converts to DC voltage then connected to

the different loads.it also discusses how the outputs voltage can be varied.

The fourth chapter is discussing and displaying all the results obtained, the analysis of the

results and the limitations encountered in the project.

Chapter five will discuss on the conclusion and summary of the development of the three phase

controlled rectifier project. In this chapter also discusses on the recommendation for this project

development or modification.

6

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter presents an overview of rectifier, the projects that have been made from various

sources like journals, books, articles and others. From the literature review the input that has

been collected is useful for better understanding of this project. It is because for nearly a

century, rectifier circuits have been the most common power electronics circuits used to convert

AC to DC. The AC-DC converter produces a DC output from an AC input while the average

power transferred from an AC source to a DC load.

A rectifier is an electrical device that converts alternating current (AC), which periodically

reverses direction to direct current (DC). The word rectification is used not because these

circuits produce DC but rather because the current flows in one direction. Rectifiers have many

uses including as components of power supplies and as detectors of radio signals. Rectifiers

may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other

components.

Rectifiers are used as stand-alone units feeding single and multiple dc loads and as input stages

of ac systems because of their virtually unlimited output electromechanical transients occurring

in motor drives and power suppliers.AC/DC line –commutated converters or called natural

commutation or passive rectifiers, are the most usual choice for applications, where a single

phase and three phase supply is available. This is due to circuits requiring a minimum number

of active and passive components. Thyristors are the main line-commutated power switches.

Line commutation is the transfer of current from one conduction element to the next, as a

function of the mains voltage .to turn on a thyristors an injection of a current pulse into its gate

is required.

In low power applications, vehicle, medicine and household devices where there is no ac supply

or where reactive current and harmonics caused by line commutation would be unreachable we

employ forced commutated converters having a more complex circuitry and involving higher

losses sometimes. For this purpose active rectifiers are developed.

Three-phase controlled rectifiers have a wide range of applications, from small rectifiers to

large High Voltage Direct Current (HVDC) transmission systems. They are used for electro-

chemical process, many kinds of motor drives and speed control, traction equipment, controlled

power supplies, and many other applications.

From the point of view of the commutation process rectifier circuits are divided broadly into

two types of AC-DC converters which are half-wave rectifiers and full-wave rectifiers, half-

wave describes the current in each a.c line is unidirectional .The full-wave circuits are which

are in effect two half-wave circuits in series, one feeding into the load the other returning load

current directly to the a.c lines eliminating the need of a.c neutral, the full-wave are commonly

called bridge circuits alternatively double-way circuits. Which then are further subdivided into

uncontrolled, semi-controlled and fully controlled. The input of these converters can be single

phase or multi-phase (3 phase). The systems built on diodes are the uncontrolled rectifiers since

the output cannot be changed while the controlled rectifiers are those built on thyristors and

7

power transistors their dc output can be changed. The fully-controlled are referred as

bidirectional converters as it permits power flow in either direction between supply and load.

The semi-controlled rectifiers contain a mixture of thyristors and diodes which prevent a

reversal of load voltage but do allow adjustment of the direct (mean) voltage level. The semi-

controlled and uncontrolled (diodes only) are called unidirectional converters as they permit

power flow only from a.c supply to d.c load.

They are further divided according to their inputs either single –phase or multiple (three) phase.

Pulse-number is a manner of describing the outputs characteristics of a given circuit and defines

the repetition rate in the direct voltage waveform over one cycle of the a.c supply.

2.2 Theory of rectifiers

Half-wave rectifier

o Uncontrolled rectifiers

Single phase uncontrolled rectifiers

Three phase controlled rectifiers

o Controlled rectifiers

Single phase controlled rectifiers

Three phase controlled rectifiers

Full-wave rectifier

o Uncontrolled rectifiers

Single phase uncontrolled rectifiers

Three phase controlled rectifiers

o Controlled rectifiers

Single phase controlled rectifiers

Three phase controlled rectifiers

Assumption made is all voltages at the input of the rectifiers have sinusoidal waveforms with

period Tmains = 20 ms (corresponding to fmains = 50 Hz). With the usual definition

𝜔 = 2𝜋𝑓 =2𝜋

𝑇

Uncontrolled rectifiers can be classified based on the type of input power supply as

• Single phase uncontrolled Rectifiers which operate from single phase ac input power supply

and they are further subdivided into different types

• Half wave uncontrolled rectifier which uses a single diode device

• Full wave uncontrolled rectifiers

8

Half wave uncontrolled rectifier which uses a single diode device (which provides

output one half cycle of input ac supply, and it provides low dc output).Full wave uncontrolled

rectifiers (which provide higher dc output).

Full wave controlled rectifier using a center tapped transformer (which requires two diode).

• Single phase controlled Rectifiers which operate from single phase ac input power supply and

they are further subdivided into different types

• Half wave controlled rectifier which uses a single diode device

• Full wave controlled rectifiers

Half wave controlled rectifier which uses a single thyristor device (which provides output

control only in one half cycle of input ac supply, and it provides low dc output).Full wave

controlled rectifiers which provide higher dc output.

Single phase semi-converter (half controlled bridge converter, using two SCR’s and two

diodes, to provide single quadrant operation).Single phase full converter (fully controlled

bridge converter which requires four SCR’s, to provide two quadrant operation. Full wave

controlled rectifiers which provide higher dc output.

Full wave controlled rectifier using a center tapped transformer which requires two thyristors.

Figure 6:uncontrolled rectifier and sub divisions

9

Figure 7 : controlled rectifier and sub divisions

10

2.2.1 Single-Phase Rectifiers

In applications, the DC voltage from the rectifier is connected to a DC bus for

distribution to several different circuit systems, subsystems and other converters as loads. In

other applications, the rectifiers also supply power to inductive-resistive (motors) and

capacitive-resistive (power supplies) loads.

2.2.1.1 Single-Phase uncontrolled Half-Wave Rectifiers

A single-phase uncontrolled half-wave rectifier consists of a single diode .It produces an output

waveform that is half of the incoming AC voltage waveform. The positive pulse output

waveform shown in Figure.8 occurs because of the forward-bias condition of the diode.

Over the negative portion of the input waveform, the diode is reverse biased ideally so no

current flows. Thus, the output waveform is zero or nearly zero during this portion of the input

waveform. By using diode, the DC level of the output and the power transferred to the load are

fixed when the source and load parameters are established. It produces an output waveform

that is half of the incoming AC voltage waveform as shown im

Figure 8: Single-phase half wave diode rectifier-R/L Load

(2.1)

(2.2)

11

2.2.1.2 Single-Phase controlled Half-Wave Rectifiers

Unlike the diode, the silicon controlled rectifier (SCR) will not to begin to conduct as soon as

the source becomes positive . Gate trigger current is the minimum current required to switch

silicon controlled rectifiers from the off-state to the on-state at the specified off-state voltage

and temperature. Once the SCR is conducting, the gate current can be removed and the SCR

remains on until the current goes to zero . Figure 9 shows a basic controlled half-wave rectifier.

Figure 9: A basic half wave controlled rectifier

2.2.1.3: Single Phase Full-Wave Rectifiers

A single-phase full-wave rectifier consists of four diodes arranged as shown in figure 10 and

figure 11 are a bridge rectifier and the center-tapped rectifier. This rectifier circuit produces an

output waveform that is the positive half of the incoming AC voltage waveform and the

inverted negative half.

Figure10 a: The Bridge Rectifier Figure 10 b: single diode rectifier waveforms

12

Figure 10 c: The center-Tapped Transformer rectifier

The output voltage of single phase uncontrolled rectifier in eqn(2.3)

(2.3)

The lower peak diode voltage in the bridge rectifier which consists of four diodes arranged

makes it more suitable for high-voltage applications. Thus, the center-tapped transformer

rectifier in addition to including electrical isolation has only one diode voltage drop between

the source and load making it desirable for low-voltage and high current applications.

Figure 11: a full wave controlled bridge rectifier

Similar to the diode bridge rectifier topology, a versatile method of controlling the output of a

full-wave rectifier is to substitute controlled switches such as SCRs for the diode. Because of

their unique ability to be controlled, the output voltage and hence the power can be controlled

to desire levels. The triggering of the thyristor has to be synchronized with the input sinusoidal

voltage in an AC to DC rectifier circuit. The delay angle α is the angle interval between the

forward biasing of the SCR and the gate signal application Otherwise, if the delay angle is zero,

the rectifiers behave exactly like uncontrolled rectifiers with diodes. Figure 11 shows a basic

controlled full-wave rectifier.

(2.4)

13

2.2.2 Three Phase Rectifiers

Three phase rectifiers are commonly used in industry to produce a DC voltage and current for

large loads.In industrial applications, where three-phase AC voltages are available, three phase

rectifiers are favorable. At the output of this type of rectifier compared to the single-phase

rectifiers, the DC component is higher, the ripple of the output voltage is lower, and the output

power is higher. Like single phase rectifiers, three phase rectifiers also have two types that are

uncontrolled and controlled.

Three phase rectifiers are more commonly used because of the following reasons:

i. Three phase AC power is readily available.

ii. It is economical to provide DC supply to DC motors of capacity 20kW and more

from a three phase rectifier rather than single phase.

iii. The ripple frequency of the output current of the three phase rectifiers is higher than

that for single phase ones.

2.2.2.1 Three phase half-wave uncontrolled rectifiers

The use of single-way configurations one diode per phase, each diode is conducting while the

others are blocked becomes more convenient as the number of phases increases. The figure 2.8

shows a single way configuration.

Figure 12: single way rectifier

Figure 13 shows the waveforms of the phase voltages (in this example m = 3) and of the current

in the load. Each phase voltage has the same amplitude (VS) and the same frequency. There is

a phase displacement of 2𝜋/m electrical radians between one voltage and the next. In one period

there is a specific number of peaks (usually called pulses), depending on the number of phases

and on the structure of the rectifier. The number of pulses in a period is indicated by p.

14

Figure 13: waveforms for the number of pulses of single way rectifier

2.2.2.2 Three phase full wave uncontrolled rectifiers

Three phase full-bridge rectifier use diodes as switch. A simplified circuit by assuming an

idealized drive and by replacing a real load by a current generator. Due to neglecting the series

inductances of the driving generators, three phase rectifier divides into two groups which are

top group and bottom group. For top group, diode with its anode at the highest potential will

conduct at one time. The other two will be reversed. Thus for bottom group, diode with its

cathode at the lowest potential will conduct. The other two will be reversed. Figure 2.15 a

shows the phase voltage and the resulting combinations of line-to-line voltages from a balanced

three phase source. The output voltage is equal to the difference of the voltages at the points P

and N compared to the neutral point n,

𝑉𝑜 = 𝑉𝑝𝑛 − 𝑉𝑁𝑛 (2.5)

Since one diode of the upper group and one diode of the lower group are always conducting,

the voltages VPn and VNn are equal to one of the AC voltages Van,Vbn or Vcn . So, the output

voltage follows in turn six sinusoidal voltage during one cycle as shown in Figure 14 b these

been 𝑉𝑎 − 𝑉𝑏, 𝑉𝑎 − 𝑉𝑐, 𝑉𝑏 − 𝑉𝑐, 𝑉𝑏 − 𝑉𝑎, 𝑉𝑐 − 𝑉𝑎, 𝑉𝑐 − 𝑉𝑏 all having the maximum value

of the line voltage that is √3 times the phase voltage and for this reason, these rectifiers are

often called the six-pulse rectifiers. The output voltage during one cycle practically consists of

six sinusoidal peaks thus its ripple is small and the average value is close to the amplitude of

the line voltages. The phase currents are determined by

Figure 14: Three - phase bridge rectifier (a) and (b) it’s ideal equivalent circuit

15

Figure 15(a): source and output voltage

Figure 16 b: waveforms of voltages and current of a three phase rectifier operating in the constant load current mode

The maximum reverse voltage of a diode is equal to the amplitude of the line voltage. The

output voltage is cyclic with the cycle π/3. The corresponding Fourier series is

16

(2.6)

Where the average value is

𝑉𝑎𝑣𝑟 = 2 ×3√3

2𝜋 Vph (max) (2.7)

Or calculated from the six-pulse load voltage waveform which is

𝑉𝑚𝑒𝑎𝑛 =3

𝜋𝑉𝑙𝑖𝑛𝑒(𝑚𝑎𝑥) (2.8)

Therefore, the average value of the output voltage is approximately equal to the amplitude of

the line voltage, 𝑉𝑙𝑖𝑛𝑒(𝑚𝑎𝑥)

The diode current waveforms shown in figure 2.11b reveal that each diode conducts the full-

load currents for one third of a cycle that is 𝜋

3𝑜𝑓 𝑡ℎ𝑒 𝑐𝑦𝑐𝑙𝑒, the order of commutation

determining the numbering of the diodes in circuit. Each diode conducts during 120°. They

conduct in pairs (6, 1), (1, 2), (2, 3), (3, 4), (4, 5), (5, 6), and (6, 1). Diodes turn on in the

sequence 1, 2, 3, 4, 5, 6, 1…

2.2.2.3 Three phase Controlled Rectifiers

Similar to single phase controlled rectifier, the output of the three phase rectifier can be

controlled by replacing the diodes with thyristors (SCRs). Figure. 17 a shows the controlled

six-pulse three phase rectifier. Thyristor is a device whose transition from the blocking to the

conducting state depends not only on the polarity of the anode–cathode voltage as for diodes,

which are naturally commutating devices but is also controlled via the application of an

adequate current pulse at the gate terminal. Thyristors have three terminals the trigger pulse is

applied to the gate while the anode–cathode voltage is positive. The difference in the principle

of operation is that the diodes are conducting when forward biased and the thyristors conducts

if a triggering pulse is present in the gate circuit together with the forward bias, this is the way

of controlling the angle of conduction and consequently the average value of the output voltage.

Thus, the transition of the output voltage to the maximum instantaneous line-to-line source

voltage can be delayed. The delay angle α is the interval between when the SCR becomes

forward-biased and when the gate signal is applied.

In this circuit there is a problem which is the one of starting .when connected to the three phase

a.c. supply firing gate pulses will be delivered to the thyristors and it must be in the correct

sequence but if only a single firing gate pulse is used no current will flow as the other thyristor

in the current path will be in the off-state. In order to start the circuit functioning two thyristors

must be fired at the same time in order to commence current flow. From the supply of three

phase when Va is at its peak value or its more positive the next firing pulse will be to

thyristor T2 however thyristor T2 will not conduct unless at the same time T1 is pulsed.

Reference to the waveforms shown in fig.17 b this shows that there are two thyristors

conducting at that instance. Hence for starting purposes the firing circuit must produce a firing

pulse 60° after its first pulse .Once the circuit is running normally the second pulse will have

no effect as the thyristor will already be in the on-state.

17

The starting pulse can be fed to the thyristors by two methods;

i. Each firing circuit having two isolated outputs one to its own thyristor and the other to

the previous thyristor.

ii. The circuit can be electronically linked so that when each firing circuit initiates a pulse

to its own thyristor it also does likewise to the previous firing circuit.

A large firing delay causes the load voltage to have negative periods as shown in figure 17 .c

and makes it difficult to visualize the load voltage waveform from the two three-phase pictures

hence the six-line voltages 𝑉𝑎 − 𝑉𝑏, 𝑉𝑎 − 𝑉𝑐, 𝑉𝑏 − 𝑉𝑐, 𝑉𝑏 − 𝑉𝑎, 𝑉𝑐 − 𝑉𝑎, 𝑉𝑐 − 𝑉𝑏 gives a

direct picture of the load voltage waveforms and clearly shows that zero mean voltage is

reached when firing delay angle is 90°. Equation (2.4) is valid when the condition of continuous

conduction that is the instantaneous voltage at the DC terminals is at all times positive is

satisfied. For delay angles beyond 60 degrees the instantaneous voltage at the DC terminals

goes to zero and negative if the load has an inductive component and the current does not flow

continuously anymore.

The average output voltage is determined by

𝒇𝒐𝒓 𝟎 <∝< 𝝅/𝟑 (2.9)

Equation (2.4) shows that the average output voltage is reduced as the delay angle increases.

Compared to Vo of the three-phase diode rectifier the factor cosα is present here. If the angle

of triggering is greater than π/3 (α > π/3), the rectifier operates in the discontinuous mode of

the load current. Then the average value of the output voltage is determined by

𝒇𝒐𝒓 𝝅/𝟑 <∝< 𝟐𝝅/𝟑 (2.10)

18

(b)

Figure 17: (a) fully-controlled three phase bridge ciruit, (b) the output voltage waveforms with small fring angle and (c) with large firing angle

2.3 Thyristor commutation — effect of the AC input reactance

The commutation of real devices is not instantaneous. Turning on and off a diode or a thyristor

takes a

finite time. Taking into consideration the thyristors only, turning off occurs when the anode

current goes below holding current Ih. In addition to the finite turn off time, the inductance of

the line and of the secondary of the transformer plays an important role in rectifier operation.

The commutation between the phases takes a finite time, during which the two phases involved

are almost shorted. This time (electrical angle) is called commutating or overlapping time

(angle) and it is usually indicated as µ during the transition from phase B to phase A, both

thyristors T2 (which is turning off) and T1 (which is turning on) are conducting at the same

time. The current isc is limited, in practice, by the impedance seen at the AC input of the

19

rectifier, here indicated as LS.

Figure 18: Commutation of thyristor from one phase to another

Assuming the dc current ID to be smooth, and with the help of figure 18, the following relation

is deducted:

(2.20)

where isc is the current in the valve being fired during the commutation process (thyristor T2

in figure 18). This current can be evaluated, and it yields:

𝐼𝑠𝑐 =√2

2𝐿𝑠. 𝑉𝑓 − 𝑓

𝑐𝑜𝑠𝜔𝑡

𝜔+ 𝐶 (2.11)

The constant “C” is evaluated through initial conditions at the instant when T2 is ignited. In

terms of angle, when wt=α:

Where wt=α, 𝐼𝑠𝑐 = 0

Therefore

𝐶 =𝑉𝑓−𝑓

2𝐿𝑠. 𝑐𝑜𝑠 α (2.12)

During the commutation time, the load current ID remains constant, isc returns through T1,

and T1 is automatically switched-off when the current isc reaches the value of ID. This happens

because thyristors cannot conduct in reverse direction. At this moment, the overlap time lasts

µ, and the current ID is then conducted by T2. In terms of angle:

20

For a given direct current ID and the corresponding delay angle it can be calculated from the

following equation:

(2.13)

The fact that during the commutation time two switches are conducting at the same time creates

short circuit and a reduction in the rectified voltage (vD) and in its average (VD). The reduction

in

the DC voltage by an amount Vmed in Figure. 2.15.

Figure 19: Effect of the overlap angle on the voltage and currents

The area Vmed showed in figure 19 represents the loss of voltage that affects the average

voltage VC, and can be evaluated through the integration of v during the overlap angle µ. The

voltage drop v can be expressed as:

(2.14)

Integrating eq. (2.14) into the corresponding period (60°) and interval (µ), at the instant when

the commutation begins (α):

(2.15)

(2.16)

21

As can be seen in Figure 2.15, the waveform of the rectified voltage vD is additionally distorted

during the overlapping angle, worsening the output ripple of the rectifier.

(2.17)

(2.18)

The load current can be written as a function of the primary winding of the transformer, if any.

(2.19)

(2.20)

Where a=Vf-fsec/Vf-fprim.

2.4 Power displacement factor and Power factor

The angular displacement between the fundamental components of the.AC line current and the

associated line to neutral voltage.The Displacement Power Factor is defined as

DPF = cos1 (2.21)

cos = 𝑐𝑜𝑠𝛼

In the case of non-sinusoidal current, the active power delivered per phase by the sinusoidal

supply is

𝑃 =1

𝑇∫ 𝑣𝑎(𝑡)𝑖𝑎(𝑡)𝑑𝑡

𝑇

0 = 𝑉𝑎

𝑟𝑚𝑠𝐼𝑎𝑟𝑚𝑠cos1 (2.22)

Where Vrms is the rms value of the voltage va, and Ia 1rms the rms value of ia1 (fundamental

component of ia). Analog relations can be obtained for vb and vc.

The apparent power per phase is given by:

(2.23)

The power factor is defined by:

(2.24)

22

Where PF becomes:

(2.25)

This equation shows clearly that due to the non-sinusoidal waveform of the currents, the power

factor of the rectifier is negatively affected both by the firing angle α and by the distortion of

the input current. In effect, an increase in the distortion of the current produces an increase in

the value of Iarms in equation (2.24), which deteriorates the power factor.

Considering now the r.m.s. value of the distortion component in the line current,

(2.26)

The total harmonic distortion is given as

(2.27)

2.5 Effects on the AC mains voltage

During the commutation of the thyristors the two phases involved are almost shorted through

the line/transformer secondary reactance. This causes notches on the AC voltage. It can be

demonstrated that these notches have a maximum depth and width depending on the delay

angle α ,the line/transformer secondary reactance LS, the phase-to-phase voltage Vf-f, and the

average value of the rectified current ID.

(2.28)

The total harmonic distortion of the mains AC voltage can be calculated from the impedance

of the

AC source the line feeding the primary of the rectifier’s transformer LSline and the harmonics

of the converter’s input current. The THD can thus be given by

(2.29)

23

2.6 How to reduce the harmonics on the AC mains

Various techniques of improving the input current waveform are discussed below. The intent

of all techniques is to make the input current more continuous so as to reduce the overall current

harmonic distortion. The different techniques can be classified into four broad categories:

i. Introduction of line reactors and/or DC link chokes

ii. Passive filters (series, shunt, and low pass broadband filters)

iii. Phase multiplication (12-pulse, 18-pulse rectifier systems)

iv. Active harmonic compensation

The term three-phase line reactor or just reactor is used in the following paragraphs to denote

three-phase line inductors.

2.6.1 Three-Phase Line Reactors

Line reactors offer a significant magnitude of inductance that can alter the way the current is

drawn by a nonlinear load such as a rectifier bridge. The reactor makes the current waveform

less discontinuous, resulting in lower current harmonics. Since the reactor impedance increases

with frequency, it offers larger impedance to the flow of higher order harmonic currents.

Therefore, it is instrumental in impeding higher frequency current components while allowing

the fundamental frequency component to pass through with relative ease. On knowing the input

reactance value, one can estimate the expected current harmonic distortion. A table illustrating

the typically expected input current harmonics for various amounts of input reactance is shown

in Table 1.

Input reactance is determined by the accumulated impedance of the AC reactor, DC link choke

(if used), input transformer, and cable impedance. To maximize the input reactance while

minimizing AC

Table 1: percent Harmonics vs Total line impedances

Voltage drop, one can combine the use of both AC-input reactors and DC link chokes. One can

approximate the total effective reactance and view the expected harmonic current distortion

from Table 1. The effective impedance value in percent is based on the actual loading and is:

(2.30)

Where Iact (fnd.) is the fundamental value of the actual load current and VL−L is the line-line

voltage. The effective impedance of the transformer as seen from the nonlinear load is:

24

(2.31)

Where Zeff,x-mer is the effective impedance of the transformer as viewed from the nonlinear

load end; Zx-mer is the nameplate impedance of the transformer; and Ir is the nameplate rated

current of the transformer

2.6.2 DC-Link Choke

Based on the above discussion, it can be noted that any inductor of adequate value placed

between the AC

source and the DC bus capacitor of the ASD will help in improving the current waveform.

These observations lead to the introduction of a DC-link choke, which is electrically present

after the thyristor rectifier and before the DC bus capacitor. The DC-link choke performs very

similar to the three-phase line inductance. The ripple frequency that the DC-link choke has to

handle is six times the input AC frequency for a six-pulse ASD. However, the magnitude of

the ripple current is small. One can show that the effective impedance offered by a DC-link

choke is approximately half of that offered by a three-phase AC inductor. In other words, a 6%

DC-link choke is equivalent to a 3% AC inductor from an impedance viewpoint. This can be

mathematically derived equating AC side power flow to DC side power flow as follows:

𝑃𝑎𝑐 = 3 ×𝑉𝐿−𝑁

2

𝑅𝑎𝑐; 𝑃𝑎𝑐 = 𝑃𝑑𝑐 (2.32)

VL −N is the line-neutral voltage at the input to the three-phase rectifier.

𝑃𝑑𝑐 =𝑉𝐷𝐶

2

𝑅𝑑𝑐; (2.33)

𝑉𝑜 =3×√2×𝑉𝐿−𝐿×𝐶𝑂𝑆(𝛼)

𝜋=

3×√2×𝑉𝐿−𝑁×𝐶𝑂𝑆(𝛼)

𝜋 (2.34)

Hence 𝑅𝑑𝑐 = 2(9

𝜋2)𝑅𝑎𝑐 (2.35)

2.6.3 Passive Filters

Passive filters consist of passive components like inductors, capacitors, and resistors arranged

in a predetermined fashion either to attenuate the flow of harmonic components through them

or to shunt the harmonic component into them. Passive filters can be of many types. Some

popular ones are series passive filters, shunt passive filters, and low-pass broadband passive

filters. Series and shunt passive filters are effective only in the narrow proximity of the

frequency at which they are tuned. Low-pass broadband passive filters have a broader

bandwidth and attenuate almost all harmonics above their cutoff frequency.passive filter

components can interact with existing transformers and power factor correcting capacitors

and could create electrical instability by introducing resonance into the system. Some forms

of low-pass broadband passive filters do not contribute to resonance but they are bulky,

expensive, and occupy space. A typical low-pass broadband filter structure popularly

employed by users of ASDs .

25

2.6.4 DC side harmonic filtering

The fundamental ripple frequency is given by;

(2.36)

where p is the number of pulses.

The ripple voltage at the output of the rectifier can be represented as an independent time

varying voltage over/imposed on the average value of the rectified voltage (VDC). In order to

reduce the ripple amplitude, a passive LC filter is usually connected between the rectifier and

the load. The time-varying voltage can thus be interpreted as a composition of harmonics that

are individually weakened by the filter.

passive LC filters are used to reduce the harmonics content on the output

of the rectifiers: “a combination of L and C produces a lower ripple with normal components

values than is possible with either L or C alone” . The inductance smoothens the oscillations

in the current and the capacitance those in the voltage. The resonance frequency is given by

(2.37)

and it has to be chosen so as to satisfy the condition f0 << fr; furthermore, for a given f0, a

degree of freedom remains in the choice of L or C.

The critical inductance is calculated with the condition that the peak of the fundamental

harmonics of the current ripple through the filter inductance is equal to the minimum IDC in

the load.

(2.38)

where V ripple_1 is the amplitude of the fundamental harmonic of the ripple at frequency

fr_1. In order to avoid resonances, a damping resistor is normally added in the filter structure.

A typical scheme for a damped LC filter is given in Fig. .

26

Figure 20: passive filter

The resistance is chosen so as to limit the overshoot in the step response of the filter. From

the equation of the systems of the second order, with an overshoot equal to 1.5, the equations

for this type of filter are reported in (eqn 2.39 )

(2.39)

2.7 Silicon Controlled Rectifier

The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type

material. They act as a switch, conducting when their gate receives a current pulse, and continue

to conduct for as long as they are forward biased (that is, as long as the voltage across the

device has not reversed).

Some sources define silicon controlled rectifiers and thyristors as synonymous; others define

SCRs as a subset of thyristors, along with gate turn-off thyristor (GTO), triode ac switch (triac),

static induction transistor (SIT), static induction thyristor (SITH) and MOS-controlled thyristor

(MCT). Among the latter, the International Electro technical Commission 60747-6 standard

27

stands out. Non-SCR thyristors include devices with more than four layers, such as triacs and

DBGTOs.

The thyristor is a four-layer semiconducting device, with each layer consisting of alternately

N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and

cathode, are across the full four layers, and the control terminal, called the gate, is attached to

p-type material near to the cathode. (A variant called an SCS Silicon Controlled Switch brings

all four layers out to terminals.) The operation of a thyristor can be understood in terms of a

pair of tightly coupled Bipolar Junction Transistors, arranged to cause the self-latching action:

Figure 21: two- transistor model of the thyristor (SCR) (a and b) and its symbol

2.7.1 Working of SCR

Thyristor act in three states:

Reverse blocking mode -- Voltage applied in the direction that would be blocked by a diode .

Forward blocking mode -- Voltage applied in the direction that would cause a diode to conduct,

but the thyristor has not yet been triggered into conduction.

Forward conducting mode -- The thyristor has been triggered into conduction and will remain

conducting until the forward current drops below a threshold value known as the "holding

current"

Figure 22: layer diagram of thyristor

When the anode is at a positive potential VAK with respect to the cathode with no voltage

applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased.

As J2 is reverse biased, no conduction takes place (Off state). Two ways of switching On SCR.

If VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown

of J2 takes place and the thyristor starts conducting (On state).

28

If a positive potential VG is applied at the gate terminal with respect to the cathode, the

breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate

value of VG, the thyristor can be switched into the on state immediately. It should be noted

that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective

of the gate voltage, until either: (a) the potential VG is removed or (b) the current through the

device (anode−cathode) is less than the holding current specified by the manufacturer. Hence

VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.[6]

These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger

current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it

is evident that there is a minimum gate charge required to trigger the thyristor.[2,6]

2.7.2 Switching Characteristics of SCR

In a conventional thyristor, once it has been switched on by the gate terminal, the device

remains latched in the on-state (i.e. does not need a continuous supply of gate current to

conduct), providing the anode current has exceeded the latching current (IL). The thyristor will

remain in the on state (the region of low resistance) as long as the anode current is higher than

the holding current, IH . The holding current is the lowest anode current maintaining the

thyristor in the state of conduction with the gate open (IG = 0) [6, 5].

Figure 23: The static V-I characteristic of an SCR

A thyristor can be switched off if the external circuit causes the anode to become negatively

biased. In some applications this is done by switching a second thyristor to discharge a

29

capacitor into the cathode of the first thyristor. This method is called forced commutation. After

a thyristor has been switched off by forced commutation, a finite time delay must have elapsed

before the anode can be positively biased in the off-state. This minimum delay is called the

circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time

causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons)

that have not yet recombined [6].

For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60

Hz), thyristors with lower values of tQ are required. Such fast thyristors are made by diffusing

into the silicon heavy metals ions such as gold or platinum which act as charge combination

centers. Alternatively, fast thyristors may be made by neutron irradiation of the silicon [6, 7].

2.7.3 SCR Ratings

A data sheet for a typical thyristor follows this section and includes the following

information:

Surge Current Rating (IFM)—The surge current rating (IFM) of an SCR is the peak anode

current a SCR can handle for a short duration.

Latching Current (IL) — a minimum anode current must flow through the SCR in order for it

to stay ON initially after the gate signal is removed. This current is called the latching current

(IL).

Holding Current (IH) — After the SCR is latched on, a certain minimum value of anode

current is needed to maintain conduction. If the anode current is reduced below this minimum

value, the SCR will turn OFF.

Peak Repetitive Reverse Voltage (VRRM)—the maximum instantaneous voltage that an SCR

can withstand, without breakdown, in the reverse direction.

Peak Repetitive Forward Blocking Voltage (VDRM)—the maximum instantaneous voltage

that the SCR can block in the forward direction. If the VDRM rating is exceeded, the SCR

will conduct without a gate voltage.

Non-repetitive Peak Reverse Voltage (VRSM) — the maximum transient reverse voltage that

the SCR can withstand.

Maximum Gate Trigger Current (IGTM) — the maximum DC gate current allowed to turn

the SCR ON.

Minimum Gate Trigger Voltage (VGT) — The minimum DC gate-to-cathode voltage

required to trigger the SCR.

Minimum Gate Trigger Current (IGT) — The minimum DC gate current necessary to turn the

SCR ON .

2.7.4 Gate Circuit Requirements.

The trigger signal should have voltage amplitude greater than the minimum gate trigger

voltage of the thyristor being turned ON. It should not be greater than the maximum gate

trigger voltage, either. The gate current should likewise be in between the minimum and

maximum values specified by the thyristor manufacturer. Low gate current driver circuits can

fail to turn ON the thyristor [7].

The thyristor is a current controlled switch and so the gate circuit should be able to provide

the needed turn ON gate current into the thyristor. Sufficient gate trigger current will turn ON

the thyristor and current will flow from the anode to the cathode provided that the thyristor is

favorably biased and the load is such that the current flowing is higher than the latching current

30

of the thyristor. The gate trigger will turn ON the thyristor only if it occurs during the positive

part of the AC .Any trigger signal during the negative part of the AC cycle will not turn ON

the thyristor and the thyristor will remain in blocking state. Keeping the gate signal ON during

the negative part of the AC cycle does not typically damage a thyristor [7].

The current into and the voltage at, the gate are both subject to maximum values, but turn-on

requirements demands they also exceed certain minimum levels. The product of gate and

current gives a power level to which a maximum is set.in figure 2.19 shows the limits imposed

on the gate cathode characteristic, giving the area into which must be fitted the gate firing signal

for certain firing into the on-state to take place. Final stage of gate triggering network consist

of a pulse transformer for isolation ,a resistor (R1) to limit the gate current and R2 to limit the

gate voltage when the thyristor is in the off state.[1]

(a)

(b)

Figure 24: (a) Firing network desirable network connection,(b) firing network load line

31

2.7.5 SCR protection

2.7.5.1 Overcurrent

The current rating of a device is the current which raises the temperature of the junction to its

top limit (normally around 125°C). An overcurrent will raise the temperature of the junction

excessively and cause malfunctions or the destruction of the device.

The simplest way to protect a thyristor is using adequate fuses. They must be fast acting fuses

preventing the rise to high arc voltages (less than 1.5 times the peak voltage in circuit). The

I2t parameter normally characterizes fuses: this value must be lower than the I2t that would

damage the thyristor (the semiconductor manufacturer usually indicates the maximum I2t for

the protection fuses).

2.7.5.2 Overvoltage

Withstanding the estimated reverse voltage for the application where it is used is one of the

main parameters for the design of the converter. If the thyristor is submitted to a reverse

voltage greater than its rated value, it will break down. Choosing oversized devices (VRRM

30% or 50% higher than that one expected) is a solution in this case.

2.7.5.3 Transients/ surges

Voltage transients or voltage surges, i.e., an excessive slew rate of voltage (dv/dt) are another

source of overvoltage that may damage the thyristors. Transients may originate from sources

that are either internal or external to the device.

The general approach to protect thyristors from voltage surges is to quickly store the surge

energy in a capacitor, and then to dissipate it slowly in a resistor. Each thyristor commutation

causes some transient voltage peaks, in particular at turn off. Owing to the presence of an

inductance (line, transformer winding, etc.) in series during its conduction phase, a high peak

reverse voltage is generated when the thyristor is turned off. In order to mitigate this voltage

surge, a RC combination, called snubber circuit, is connected in parallel to each device.

Capacitor C1 suppresses the voltage surge dv/dt that appears when the thyristor Th goes into

the blocking state. Resistor R (R1) is used to damp possible oscillations in the LC circuit (L is

the inductance of the AC connection seen by the thyristor). The same resistance R (or, in the

circuit on the right, R2) has to limit the discharge current from the capacitor through the

thyristor when it starts to conduct again.

32

Typical values for C are 0.1–1 μF and for R are 10–1000 Ω

Figure 25: protecting thyristors from internal surges using snubber circuit

External transients come from the AC supply line. The main cause is the action of the power

converter’s main contactor; when it opens, it interrupts the magnetizing current at the primary of the

transformer. The energy stored in the secondary windings of the transformer is then dissipated

throughthe thyristors and the load. When the contactor closes, a voltage overshoot may occur in the

oscillating circuit constituted by the inductance of the secondary windings and a capacitance, either

stray or physically present. Also in this case a RC combination is used. The capacitance must be able

to store the energy of the transformer. Sometimes the RC groups are connected directly between the

phases immediately before the rectifier. In order to decouple the capacitance from the inductance of

the connection, a bucket circuit is often preferred. In practice the RC group is connected to the line

through a diode rectifier (Fig. 26). Resistance R1 is the damping resistance, calculated from the

inductance of the line and of the transformer and the capacitance C. Resistance R2 is the discharge

resistor of the capacitor; it is sized in order to have a time constant of about 100 ms.

Figure 26:protecting thyristor from external voltage souces using bucket circuitCHAPTER 3

33

METHODOLOGY

3.1 introduction

This project deals with the design, development and control of a 3-phase controlled rectifier

whose power circuit is shown in Fig. 3.1 it is the most widely rectifier circuit. It consists of a

three phase transformer in star / delta connection, six thyristor bridge connected, load , CLC

filter , synchronization circuits , the trigger control and isolation circuit.

This three phase rectifier circuit must be double-pulse triggered or trigger pulse width to ensure

that at every moment it has two thyristors in same turn or triggered at same time. The three

phase transformer is connected in star / delta connection so as to reduce a multiple a multiple

of 3 harmonic currents impacting on the power and have three phase inputs which have a phase

difference of 120°.

It is a common cathode connection and is made of the three-phase half-wave phase controlled

rectifier circuits and a common anode connection of the three phase half-wave phase controlled

in series circuit.

Figure 27: Three phase controlled rectifier circuits

The trigger circuit used in this paper is that of digital triggering and the trigger circuit is

controlled by a microcontroller unit belonging to the Atmel which is an Atmega328p

microcontroller. Its main feature includes generation of 6 channel trigger pulses whose time

interval is 60 degree; only needing a synchronization signal; shift phase range of 0 degrees to

160 degree; inputting shift phase angle by variation of a potentiometer which is displayed on a

LCD display.

Each thyristor requires a trigger pulse, so this trigger circuit needs to generate 6 trigger

pulses in all and a time interval of between the trigger pulses is 60 degrees.

A block diagram of the trigger circuit is shown in Figure 1. There is a microcontroller unit

(MCU) – ATMEGA 328P from ATMEL corporation- which controls the trigger circuit to

normally run and there are also other peripheral module circuits of the MCU including LCD

display, Zero crossing detector signals and trigger pulses output.

34

Figure 28: Tri-phase control rectifier block diagram

A point on AC wave, after which it starts repeating its cycle again from 0 degrees, is called

“zero crossing point”. In order to have a full control over the entire cycle, it is necessary to

precisely find out zero crossing point. The digital controller used in this proposed design is in

the form of a microcontroller unit (MCU). MCUs, being inexpensive, fulfill one of the primary

purposes of this design that is the economy. The controller selected for this is the ATMEL

ATmega328p.

3.2 Design

3.2.1 Input

The input to be rectified is an A.C 110V supplied. Its connected to the secondary side of three

transformers connected in delta connection on the primary side and star connected on the

secondary side. For safety purpose there is a switch and fuses of 13A on every line from the

star connection.

Three phase rectifier

6 SCRS

Three phase supply

Delta/ star

transformer

Synchronization

circuit/

Zero crossing

detector circuits

(ZCD)

MCU

Controlling

angle circuit

Isolation circuit

LCD DISPLAY

DC Output

35

Figure 29: The input circuit to rectifier

The transformers specifications were;

Primary inductance = 1H

Secondary inductance = 0.1H

Coupling factor=1.0

The virtual source connected to the primary side specifications;

Amplitude volts = 78v Vrms and frequency of 50Hz.

Fuse =13A rating

Vrms =Vm

√2 (3.1)

The Vm = 78v × √2 = 110.30v (3.2)

3.2.2 The rectifier circuit

The circuit below shows the connection of both rectifier connection, the snubber circuit,

isolation circuit and the filter of the dc at the output which is connected to a resistive load.

36

Figure 30: three phase connected to the supply with the snubber circuits and isolation circuit

Scr S4015L ratings

Vrmm/Vdrr = 200v (min)

Ig = 1 mA (min) and 20 mA (max) –the gate current

Vgt = 1.5 V max the dc gate trigger voltage

Ih = 30 mA the dc holding current;gate open

The SCR in this paper are gate triggered. Using the manufacturer ratings the calculations of the

gate voltage and currents and the components specified.

3.2.2.1 Snubber circuit.

The circuit is used for high dVD

dt protection of SCR the critical rate of rise of off- voltage.

Leakage current though the scr increases with the increase in 𝑑𝑣

𝑑𝑡 that is the rate of change of

applied voltage across the thyristor. This current can turn on the device even when the gate

signal is absent which is called the 𝑑𝑣

𝑑𝑡 and is avoided by using a Snubber circuit which

consists of a capacitor in series with a resistor applied parallel with the SCR.

37

C= 0.2 u F / 250V

R =100R /2W

3.3.2.2 Opto-coupler MOC3021

If= 60mA forward current

Vr = 3V reverse voltage Vdrm = 400V

Ih = 100uA holding current.

Idrm = 100nA .

Vtm = 3V.

The function of the opto-isolator is;

Monitor high voltage

Output voltage sampling for regulation

System control micro for power ON/OFF

Ground isolating

Figure 31: MOC3021 isolator connection to SCR and interface with ATmega328p microcontroller

Opto-isolators are made up of a light emitting devices and a light sensitivity device all wrapped

up in one packet, but with no electrical connection between the two just beams of light. They

allow two circuits to exchange signals yet remain electrically isolated.

38

As seen in the figure 3.2 terminal IO3 is to a pin of the microcontroller interfaced with

MOC3021 ic pin 1 led in series, which glows to indicate a logic high pulse from the MCU

through a current limiting resistor so to that a current is flowing in internal led of the opto-ic.

Current flows from pin 1 to 2 and LED light falls on the DIAC causing pin 6 and 4 to close.

During each half cycles of the AC source current flows through gate , series resistor and

through opto-diac to the main SCR to trigger for the load to operate .

Ohms law;

𝑉 = 𝐼𝑅 (3.3)

𝑅 =𝑉

𝐼 (3.4)

The series resistor for current limiting is;

The LED must have 15mA and 1.2 V to light on the output voltage of the mcu is 5V.

𝑅 =5𝑉−1.2𝑉

0.015 = 250R (3.5)

R= 250R

3.2.3 Synchronization circuit/ Zero crossing detector circuits.

To have full control over the firing angle of the SCRs (from 0 to π radians), it is necessary to

precisely detect the zero crossing of the sinusoidal input. Schematic of the zero crossing circuit

that was implemented is shown in fig 3.4

Figure 32: zero crossing circuit with input transformer

Since the rectifier has three phase supply three of this circuits are used so as to detect the zero

points of the three ac supply helping to trigger the SCRs in the correct sequence.

Transformer used is at the input side is used for two purposes. Firstly it scales down the mains

alternating voltage to a lower and safer value up to the 6 to 9 volts rms. This gives peak voltage

of 8.46 = √2*6 volts up to 12.728 = √2*9 volts, respectively. Secondly it provides the galvanic

isolation between mains and zero crossing detection circuitry that is another key goal of the

proposed design.

39

This scaled down and isolated AC input is then scaled down further by using a resistor

consisting on R8 and R9 and they are of equal values.

Vout = (R8/R9) × Vsec (3.6)

R11 and R12 = 10k

The transformer steps down the voltage = 10.5V

The 50Hz sine wave synchronization shift voltage from a power grid is transformed to

square-wave signal with same frequency as the sine and then the square-wave signal is sent to

the interrupt pin of the MCU which can received external interrupt.

controller circuit is required to sense the zero crossing instant of the input voltage and from

thereof generates six equidistant trigger pulses in each cycle (one time period or 20 ms) for

switching of SCR of three phase converter.

The maximum voltage that the zero crossing is 4.88 v which is applicable for the mcu .

Figure 33: square wave zero crossing signals of the two phases

3.3.3 Controlling voltage

Analog Voltage (0-5 V) is used for controlling the converter power, ADC of Atmega-328

microcontroller is 10 bit resolution. Analog Voltage (0-5 V) is converted in to (0-1023) count.

Analog voltage is provided to one of the channel of ADC port. As per variation in the analog

voltage there is change of digital value. ADC conversion takes around 250 micro second. PWM

is achieved with the help of variable controlling voltage.

40

Figure 34: Controlling Voltage circuit

3.3.4 Generating of the firing pulses

A supply of 5v dc is used in this the specification of this paper was to trigger the SCRs the

controlling analog circuit varies the input from 0v to 5v.

ADC of ATmega328p is of 10-bit resolution hence the maximum value from the ADC WITH

+5 volts reference will be of 0-5 V are 0-1023 counts therefore a scaling down factor of 5.68

is used.

The 0-5v range controls the firing angle 0° to 180°

The presented results were drawn by operating the system in 50 Hz mode due to the national

standard of power transmission in Kenya.

𝑓 =1

𝑇 (3.7)

So the period of the supply is;

𝑇 =1

𝑓 (3.8)

𝑇 =1

50= 0.02 = 20𝑚𝑠𝑒𝑐 (3.9)

Half cycle time the period is 10ms the delay angle can thus be calculated as ;

𝛼 = 𝐴𝐷𝐶/5.68 (3.10)

Where ;

ADC = the output of the ADC converter/ the counts

𝛼 =is the firing angle

41

The ADC reading is converted into a delay after which firing pulse is to be generated. using

equation(3.11)

𝑑 = (𝑎𝑑𝑐 ∗ 5) ∗ 1.955 (3.11)

d - delay in microseconds

adc is the output of the ADC

1.955- value is a scaling up factor for the ADC reading

5 -is the reference value

In the circuit a 4.7V zener diode is used so as to clump the voltage not to exceed 4.3 V due to

the specifications of this paper was for the firing angle not to exceed 163° firing angle

Table 2: the delay angle calculated table

Voltage value (V) ADC counts Angle (𝜶) Delay (d) ms

0 0 0 0

1 204.6 36.02 1.9999

1.5 306.9 53.87 2.9999

3 613.8 108.06 5.9999

4.3 879.78 154.48 8.5998

4.5 920.7 162.09 8.9998

The output of ATmega328p is 4.8V and a current of 10miliampers each pin. The current is

enough to trigger the SCR used which is has a maximum of gate current 20mA.

The triggering of the 6 scrs are in sequencial form.each of the 6 SCRs are switched at an interval

of 60° sequentially,from T1,T2,T3,T4,T5,T6,and the sequence repeats again after a complete

cycle. 60° translate to a delay of 3331.32 microseconds .

From the theory part for the flow of the current two SCRs must be conducting at the same time

forming a pair of SCRs conducting.as shown in the table

Table 3: The algorith of triggering the SCR

SCR pair conducting Angle in degrees Extracted line

voltage

T6 , T1 ∝+30 to ∝+90 Vab

T1 , T2 ∝+90 to ∝+150 Vac

T2,T3 ∝+150 to ∝+210 Vbc

T3,T4 ∝+210 to ∝+270 Vba

T4,T5 ∝+270to ∝+330 Vca

T5 ,T6 ∝+330 to ∝+30 Vcb

T6,T1 ∝+30 to ∝+90 Vab

The firing angle = ∝, a phase shift of 60°

Using table 3.2 helped In the programming of the microcontroller.

42

ATmega328p has been programmed for triggering the power circuit. The execution of the

program in step by step process is explained using a flowchart. Which is show the flowchart

below.

From the output of the zero crossing circuit which is a square wave on which shows where the

AC wave crosses the zero is an input to the ATmega328p and INT01 and INT1 from the three

synchronization circuits using the Pin-change interrupts of the microcontroller causes an

interrupt showing that a phase has crossed the zero point from figure 3.5 show that at the rising

edge of the square wave that is when the phase is on the positive half.

The MCU executes its interrupt service routine (ISR).the function of the ISR is to determine

how to generate rising edge of the first trigger pulse which for elimination of the starting

problem of the rectifier and it initializes Timer 1 in the MCU to start counting from 0 and

counts up until it hits the ‘TOP’ value which the ‘TOP’ value is from the varied voltage which

gives the desired delay divided by a factor and it fires an interrupt which then T1 is fired at 30°

approximately 1.667sec the first pulse is generated.by setting PORT D , bit-3 high and a delay

of 1 millisecond and then timer runs the second time when the timer overflows the MCU

executes ISR of the timer.In this routine PORTD,bit-5 is cleared this pin is connected to T1

since the time interval between the first pulse and the second pulse is 60° which is

approximately 3.334 millisecond which means the time interval between the falling edge of the

first pulse and the rising edge of the second pulse is t1=3.334-1=2.334ms,the timer runs the

third time and when it hits the ‘TOP’ value it overflows and the MCU executes the timer ISR

in the routine PORT D,bit-5 high connected to T2 and a second pulse to T1 both pins are set to

high to start the second trigger pulse and the following steps reapeat the generation process of

the first pulse until the other pulses are outputted at this point the zero-crossing wave of phase

two has caused an interrupt. The length of time of the timer is determined by shift angle ∝

inputted though varying the potentiometer.

Here timer 1 is as a 16-bit timer and operates in mode 1. The MCU has an external 16 MHz

oscillator and fuses are set to use the external crystal clock. Fuses are used to control things

like which oscillator to use and what speed to run , blowout detection and the size of the boot

flash.

The trigger pulses are outputted in a way that each thyristor is triggered twice that is a main

pulse and secondary pulse at 60° shift from the main pulse. pin PD3 outputs a main pulse to

thyristor 1 and at the same time PB3 outputs a secondary pulse to thyristor 6;pin PD5 outputs

main pulse to thyristor 2 at the same time pin PD3 outputs the secondary pulse to thyristor 1;pin

PD6 outputs the main pulse to thyristor 3 and at the same time pin PD5 outputs the secondary

pulse to thyristor 2;pin PB1 outputs a main pulse to thyristor 4 and at the same time pin PD6 a

secondary pulse to thyristor3;pin PB2 outputs a main pulse to thyristor 5 and at same time pin

PB1 a secondary pulse to thyristor 4;pin PB3 outputs a main pulse to thyristor 6 and at the same

time pin PB2 outputs a secondary pulse to thyristor 5. And this process repeats its self in the

cycles.

43

Figure 35: MCU and LCD display

Figure 36: waveform of the output pulse

3.4 Software Design

the software design includes the main program,the display subroutine and the analog voltage

varied that is the ADC conversion,the external interrupt service routine and the timer interrupt

service routine.

3.4.1 Main Program

In this program it initializes the pins which input and the output pins; that is the analog pin(PC0)

from the controlling voltage circuit and the ADC handling initialization, three interrupt pins

(PCINT8,PCINT10,PCINT11) from the three zero crossing detector , the output of the trigger

pulses pins with their corresponding thyristor (PD3-T1, PD5-T2, PD6-T3, PB1-T4, PB2-T5,

PB3-T6) and the display subroutine.

44

NO

NO YES

NO

YES

Figure3.9 flow chart for six-pulse generation

Start

Initialization of pins and ADC

conversion

Read the analog voltage and do a conversion (store the

counts)

Is the external

interrupt detected?

Clear the PCINT11 and provide a delay of 30°

Enable interrupt bits and wait for external pulse

Check if bit 3 is high?

Is the bit

3 high? Provide a pulse width of 1ms

Clear port D, bit3 Check the pcint10?

Is the interrupt

detected?

Interrupt ISR executed and start the timer

Timer overflow and provide a 60° delay

Check if pin required bit is high?

Provide a delay for pulse

Is the pin required

bit high?

45

3.4.2 ADC program for delay

NO

YES

Figure 37: flow chart ADC subroutine and delay control

ADC program for delay control

Set the PC0 as input

Enable interrupt bits, make AD mode ON

Start the conversion

Check for completion of conversion

Call delay routine

Set the pin required bit high

Provide a delay

Clear the pin set high

Return

Has the conversion

ended?

46

CHAPTER 4:

RESULTS AND ANALYSIS

4.1 Theoretical Results

This are on ideal case basis from the equation (4.1);

𝑉𝑑𝑐 =3√2

𝜋𝑉𝑙 × cos 𝛼 for 0 <∝<60° (4.1)

𝑉𝑜 =1

𝜋3⁄

∫ 𝑉𝑙𝑙𝑚 sin(𝜔𝑡) 𝑑(𝜔𝑡)𝜋

𝜋3+∝⁄

= 3𝑉𝑙𝑙𝑚

𝜋{1 + cos(𝜋

3⁄ + 𝛼)}

𝑓𝑜𝑟 60° <∝< 120°

𝑉𝑙 = 𝑅𝑀𝑆 𝑉𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑖𝑛𝑒 − 𝑙𝑖𝑛𝑒 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 =110𝑣

√2= 77.78𝑣

Table 4: ideal results

Firing angle Dc Voltage

0° 181.93V

30° 157.56V

70° 62.23V

130° 64.99V

Table 5 Simulated results

Firing angle Dc voltage

47

CHAPTER 5

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

The project the software to trigger six thyristor or SCRs used in the power circuit of three-

phase, six pulse, and controlled rectifier has been discussed. Three fully controlled rectifiers

are very popular in many industrial applications particular in situations where power

regeneration from dc side is essential.in this the rectifier implemented was for use in the lab.

The design is isolated from electronic interference at input and output side. Power control is

possible by varying ∝ 𝑡ℎ𝑒 𝑓𝑖𝑟𝑖𝑛𝑔 𝑎𝑛𝑔𝑙𝑒 with controlling voltage with the use of ATmega328p

microcontroller and by programming the analog voltage given it controls the firing angle and

controls the dc voltage output.

The configuration used lends itself to easy series and parallel connection for increasing voltage

and current rating or improvement in harmonic behavior. Since three phase fully controlled

rectifier are obtained at the cost of increased circuit complexity due to the use of six SCRs and

their associated control circuit. This complexity can be reduced by using this software for easily

triggering the six SCRs. Moreover the trigger pulses generated by this software are

controllable.

5.2 Recommendations for Further Work

Following are areas of future study which can be considered for further research work.

1. Speed Control Of Motors

2. Transmission of HVDC

48

REFFERENCES

[1] R. Simard and V. Rajagopalan, “Economical equidistant pulse Firing scheme for

Thyristorized D.C. drives,” IEEE Trans. Ind. Electron. Contr. Instrum., vol. IECI-22, No.3, pp.

425-429, 1975

[2] Gangul M.R, G.P Jain, “Design and Implementation of AT Mega 328 microcontroller based

firing control for a tri-phase thyristor control rectifier,” International Journal of Engineering

Research & Technology (IJERT), 2013.

[3] R. Visintini, “Rectifiers,” Elettra Synchrotron Light Laboratory, Trieste, Italy

[4] Juan Dixon (Ph.D.). “Three phase controlled rectifiers,” Pontifical Universidad Católica de

Chile. Vicuña Mackenna 4860

[5] Branko Dokic, Branko Blanusa, “Power Electronics Converters and Regulators.”

[6] Mohd Helmi. “Control Rectifier for Variable Speed Single Phase DC Motor,” University

Malaysia Pahang.

[7] J. David Irwin, “The power electronics hand book,” Auburn University

[8] XU Wuxiong, “Design of Thyristor Digital Trigger Circuit Based on Microcontroller Unit,”

The 1st International Conference on Information Science and Engineering (ICISE2009)

[9] Daniel W. Hart, “power electronics,” mc craw-hill

[10] Suhel Mustajab, Mohd. Kashif Adhami, “Software based approach for Triggering 3-phase,

6-pulse, AC to DC Controlled Converter,” International Journal of Advanced Research in

Computer Engineering & Technology (IJARCET) ,Volume 1, Issue 9, 2014.

[11] Shashikant V. Lahade1 and S. R. Hirekhan2, “Microcontroller based Digital Trigger

Circuit for Converter, Proc. of Int. Conf. on Advances in Power Electronics and

Instrumentation Engineering, PEIEs

[12] R. Arockiasamy and S. Doraipandy, “A novel trigger scheme for Thyristor operating under

variable frequency anode supply,” IEEE Trans. Ind. Electron. Contr .Instrum., vol. IECI-22,

No. 1, pp. 83-85, Feb.1975.

[13] Cyril W. Lander, “power electronics,”

[14] S. Murugesan and C.Kameswara, "Simple adaptive analog and Digital trigger circuits for

thyristors working under wide range of Supply frequency,” IEEE Trans. Ind. Electron. Contr.

Instrum., vol. IECI-24, No. 1, pp.46-49, 1977.

[15] L.H.Hoang ,"A digitally controlled thyristor trigger circuit," Proc. IEEE, vol. 66, No.1,

pp. 89-91, Jan. 1978.

[16] B. Ilango, R. Krishman, R. Subramanian, and S. Sadasivam, “Firing circuit for three-phase

thyristor rectifier IEEE Trans. Ind. Electron. Contr. Instrum., vol. IECI-25, no. 1, pp. 45-49,

1978.

[17] S.C.Gupta, K.Venkatesan, and K.Eape Generalized Firing Angle Controller Using Phase

Loop for Thyrist Control,” IEEE Transactions On Industrial Electronics And Control

Instrumentation, vol. IECI-28, No. 1, February 1981

[18] John William,“introduction to solid state power electronics,” POWEREX, Inc.

200 Hillis Street Youngwood, PA 15697-1800

[19] A.W. Kelley and W.F. Yadusky, Rectifier design for minimum line-current harmonics and

maximum power factor, IEEE Trans. Power Electron. 7 (1992) 332.